how do I change one measure's metre without changing following measures?

That’s the question: how do I change one measure’s metre without changing following measures? I tried selecting the entire stack using the thing at the top (don’t know what it is called, but it allows adding, deleting & selecting of measures)?

I found an unintuitive way, which is to place a time sig in the next measure and then go back and change the focus measure. Is that the official way, so to speak?

Yes, when you insert a time signature, Dorico will re-bar the music up to the next time signature. If you only want the time signature to change for a single bar, put a “stopper” time signature in to prevent Dorico from rebarring further music.

Good, thanks. Forgive the trivial question, but what is that top line called?

The System Track.